davechidlow,
This used to be the classic way to protect fences, assuming you take the appropriate measures to protect yourself from splashes I don't think using engine oil will be any more dangerous than using creosote (which is now a banned substance because it is carcinogenic)
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Hi Dave, as Stoneyboy said, old engine used to be used on fences,
I've used it myself, it's a good way to get rid of the old oil, but keep covered up, it can be messy. Have fun!!.